The present invention relates to an improved Johnson Thermo-Electrochemical Convertor (JTEC) with integrated thermal energy storage using metal hydride materials.
The need for energy systems that are capable of both electrical energy generation and energy storage is well understood. Typically power generation systems have production profiles that are different from the energy demand profile. For example, coal power plants optimally produce power at a steady, continuous level. However, the demand for power from coal plants generally has two peaks, one in the morning and one in the evening. Demand for power during the day is higher than nighttime demand. Regarding renewable energy systems, such as solar, power generation peaks during midday and is not at all available at night. Heat energy is the dominant energy source used in electrical power generation. Electrochemical batteries are used when energy storage is required for systems that operate on heat. Such systems must first produce the electricity and then supply it to batteries for storage.
Mechanical heat engines have also been designed and used to produce electrical power. Such mechanical heat engines operate on thermodynamic cycles wherein shaft work is performed using a piston or turbine to compress a working fluid. The compression process is performed at a low temperature and, after compression, the working fluid is raised to a higher temperature. At the high temperature, the working fluid is allowed to expand against a load, such as a piston or turbine, thereby producing shaft work. A key to the operation of all engines employing a working fluid is that less work is required to compress the working fluid at low temperatures than is produced by expanding it at high temperatures. This is the case for all thermodynamic engines employing a working fluid.
For example, steam engines operate on the Rankine thermodynamic cycle, wherein water is pumped to a high pressure, and then heated to steam and expanded through a piston or turbine to perform work. Internal combustion engines operate on the Otto cycle, wherein low-temperature ambient air is compressed by a piston and then heated to very high temperatures via fuel combustion inside the cylinder. As the cycle continues, the expansion of the heated air against the piston produces more work than that consumed during the lower temperature compression process. However, the prior art mechanical devices do not achieve the high compression ratios with near constant temperature compression and expansion processes needed to approximate Carnot-equivalent cycles.
The Stirling engine was developed by Robert Stirling in 1816 to operate on the Stirling cycle in an effort to provide an engine that has high efficiency and offers greater versatility in the selection of the heat source. The ideal Stirling thermodynamic cycle is of equivalent efficiency to the ideal Carnot cycle, which defines the theoretical maximum efficiency of an engine operating on heat input at high temperatures and heat rejection at low temperatures. However, as with all mechanical engines, the Stirling engine suffers from reliability problems and efficiency losses associated with its mechanical moving parts.
In an effort to avoid the problems inherent with mechanical heat engines, Alkali Metal Thermoelectric Conversion (AMTEC) cells have been designed as a thermo-electrochemical heat engine. AMTEC heat engines utilize pressure to generate a voltage potential and electrical current by forcing an ionizable working fluid, such as sodium, through an electrochemical cell at high temperatures. The electrodes couple the electrical current to an external load. Electrical work is performed as the pressure differential across the electrolyte separator forces sodium atoms through the electrolyte. The sodium is ionized upon entering the electrolyte, thereby releasing electrons to the external circuit. On the other side of the electrolyte, the sodium ions recombine with the electrons to reconstitute sodium upon leaving the electrolyte, in much the same way as the process that occurs in battery and fuel cell type electrochemical cells. The reconstituted sodium, which is at a low pressure and a high temperature, leaves the electrochemical cell as an expanded gas. The gas is then cooled and condensed back to a liquid state. The resulting low-temperature liquid is then re-pressurized. Operation of an AMTEC engine approximates the Rankine thermodynamic cycle.
Numerous publications are available on AMTEC technology. See, for example, Conceptual design of AMTEC demonstrative system for 100 t/d garbage disposal power generating facility, Qiuya Ni et al. (Chinese Academy of Sciences, Inst. of Electrical Engineering, Beijing, China). Another representative publication is Intersociety Energy Conversion Engineering Conference and Exhibit (IECEC), 35th, Las Vegas, Nev. (Jul. 24-28, 2000), Collection of Technical Papers. Vol. 2 (A00-37701 10-44). Also see American Institute of Aeronautics and Astronautics, 190, p. 1295-1299. REPORT NUMBER(S)-AIAA Paper 2000-3032.
AMTEC heat engines suffer from reliability and efficiency issues due to the highly corrosive nature of the alkali metal working fluid. AMTEC engines also have very limited utility. Specifically, AMTEC engines can only be operated at very high temperatures because ionic conductive solid electrolytes achieve practical conductivity levels only at high temperatures. Indeed, even the low-temperature pressurization process must occur at a relatively high temperature, because the alkali metal working fluid must remain above its melt temperature at all times as it moves through the cycle. Mechanical pumps and even magneto-hydrodynamic pumps have been used to pressurize the low-temperature working fluid. Also, the AMTEC operates on a modified Rankine thermodynamic cycle that includes latent heat entropy losses and enthalpy losses which cannot be compensated. These losses include heat input for the high temperature phase change from liquid to vapor prior to expanding through the high temperature membrane electrode assembly (MEA) and exiting from the MEA as a superheated vapor only to condense at the low temperature with no work being performed.
In an effort to overcome the above-described drawbacks of conventional mechanical and thermo-electrochemical heat engines, the Johnson Thermo-Electrochemical Convertor (JTEC) system (disclosed in U.S. Pat. No. 7,160,639 filed Apr. 28, 2003) was developed.
The JTEC is a transformational technology that employs well-known principles of thermodynamics using fuel cell like MEA stacks. However, the JTEC is not a fuel cell. It does not require oxygen or a continuous fuel supply, only heat. It is a solid-state direct heat to electric conversion technology that has no moving mechanical components other than hydrogen circulation. These innovative features, in combination with operation on the Carnot-equivalent Ericsson thermodynamic cycle, represent a very significant advancement in energy conversion technology. In particular, As a system that converts heat directly into electricity, the JTEC offers revolutionary advancements in energy conversion efficiency, power density and manufacturing cost.
The JTEC operates on the Carnot equivalent Ericsson Thermodynamic cycle. It uses a first electrochemical cell operating at low temperature and coupled to a heat sink (i.e., an “electrochemical compressor” stage of the engine), a second electrochemical cell operating at high temperature and coupled to a heat source (i.e., an “electrochemical expansion” stage of the engine), and a recuperative heat exchanger that couples working fluid flow between the two cells. The JTEC includes a supply of hydrogen or oxygen as a working fluid. Working fluid is compressed in the low temperature cell and expanded in the high temperature cell whereby more work is produced during the high temperature expansion that consumed during compression in the low temperature cell. Each electrochemical cell consists of a MEA configured having a non-porous membrane that is capable of conducting ions of the working fluid and sandwiched between a pair of porous electron conductive electrodes.
In operation, working fluid passes through the MEAs by releasing an electron to the electrode on the entering side. The ions are conducted through the membrane to the opposite electrode. The electrons are coupled to the opposite electrode via an external circuit. The working fluid is reconstituted within the opposite electrode. In operation, power is applied to the low temperature cell to drive working fluid from low pressure to high pressure as heat is removed to maintain a near constant temperature compression process. The high pressure working fluid is supplied from the low temperature cell through the heat exchanger to the high temperature cell. In the high temperature cell, the process operates in reverse. Power is produced by the high temperature cell as working fluid expands through the cell from high pressure to low pressure as heat is added to maintain a near constant temperature expansion process. The resulting low pressure working fluid is supplied back to the low pressure side to the low temperature cell to continue the cycle. As in any thermodynamic engine employing a working fluid and consistent with the nature of compressible gas, a greater amount of work (electrical in this case) is extracted during high temperature expansion than the work input required for the low temperature compression. That is, the expansion process occurring at the high temperature produces enough power to drive the compression process occurring at the low temperature, as well as supply net output power to an external load.
The voltage generated by a MEA is linear with respect to temperature. The high temperature cell has a higher voltage (VHT) than the low temperature cell (VLT). Working fluid is compressed in the low temperature cell at VLT. On the other hand, working fluid is expanded in the high temperature cell at VHT as current (power) is extracted. Since the current (I), hydrogen circulation, is the same through both cells, the voltage difference means that the power generated through the expansion of hydrogen in the high temperature cell is higher than that of the low temperature cell. The power output by the high temperature cell (VHT multiplied by I) is sufficient to drive the compression process in the low temperature cell (VLT multiplied by I) as well as supply net power output to an external load ((VHT−VLT)*I)). The hydrogen circulates continuously inside the engine and is not consumed.
Ideally, a heat source and heat sink are coupled to the high and low temperature electrochemical cells, respectively, that have sufficient heat transfer to achieve near constant temperature expansion and compression, respectively. Near constant temperature compression and expansion, in combination with coupling a recuperative heat exchanger between the high and low temperature stacks to recuperate heat from fluid leaving the high temperature stack by facilitating its transfer to fluid flowing to the high temperature stack, allows the engine to approximate the thermodynamic Ericsson cycle. Less than optimum operation where the expansion and compression temperatures are not maintained nearly constant can be useful. Useful compression temperatures and useful expansion temperatures may be employed where the average expansion temperature is greater than the average compression temperature resulting in a net higher average expansion voltage than compression voltage and thereby net positive power output.
Still, with various technologies available for producing electricity from heat, the need remains for cost effective energy storage as a means for matching different energy demand profiles. Batteries are typically used to match power production profiles to demand profiles. In batteries, chemical energy is converted into electrical energy and visa-versa. For example, known electrochemical cells or batteries rely on chemical reactions wherein ions and electrons of a reactant being oxidized are transferred to the reactant being reduced via separate paths. Specifically, the electrons are transferred electrically via wiring through an external load where they perform work and the ions are conducted through an electrolyte separator. Battery type electrochemical cells add significant cost to power systems. They are typically constrained in cell size because of inherent safety and reliability problems. Lithium ion batteries, in particular, have a well established reputation of catching fire and even exploding. They can store an amount of energy that is limited by the confines of the battery casing given the amount of available reactants that may be contained therein. Very large packs of small cells are needed in order to meet the storage capacity requirements of electrical power generation systems. The packs typically require environmental control systems to maintain specific battery operating temperatures for reliability and safety. Such control systems add additional costs.
Reversible fuel cells have been developed in an effort to overcome problems associated with battery type electrochemical cells. In conventional fuel cells, the chemical reactants are continuously supplied to and removed from the electrochemical cell. In a manner similar to batteries, fuel cells produce electricity by conducting an ionized species through a selective electrolyte which generally blocks passage of electrons and non-ionized species.
The most common type of fuel cell is a hydrogen-oxygen fuel cell which passes hydrogen through one of the electrodes and oxygen through the other electrode. The hydrogen ions are conducted through the electrolyte separator to the oxygen side of the cell under the chemical reaction potential of the hydrogen and oxygen. Porous electrodes on either side of the electrolyte separator are used to couple the electrons involved in the chemical reaction to an external load via an external circuit. The electrons and hydrogen ions reconstitute hydrogen and complete the reaction with the oxygen on the oxygen side of the cell resulting in the production of water which is expelled from the system. A continuous electrical current is maintained by a continuous supply of hydrogen and oxygen to the cell.
These cells can operate in reverse to store energy by supplying water to the oxygen electrode. Power is applied to the cell to electrolyze the water in a reverse reaction to produce hydrogen and oxygen. However, there are a number of challenging liquid and gas management issues associated with operation of such cells. The MEA stacks in conventional fuel cell applications require bidirectional flow in at least one of the electrodes. For example, oxygen flow into the cathode side of hydrogen-oxygen fuel cells must be maintained as the same time that the hydrogen-oxygen reaction product, water, is exiting. As such, large flow cross-sections for fuel and the oxidizer/reaction product must be an inherent feature of the design of conventional MEA stacks for fuel cells. Cell flooding and polarization losses because of a lack of reactant with in the oxygen electrode are well established problems.
Further, fuel cell environments are very corrosive and typically require the use of an expensive noble metal catalyst (usually platinum), particularly the oxygen electrode. An even greater problem is related to the 0.4V activation energy requirement for the oxygen electrode. The electrochemical potential of a hydrogen-oxygen fuel cell is 1.2 volts. The activation voltage requirement of the oxygen electrode results in an effective output voltage of only 0.8 volts. On the other hand, when recharging or regenerating the cell, a voltage of 1.6 volts is required to overcome the reaction potential in addition to the oxygen activation voltage. Charging at 1.6 volts and discharging at 0.8 volts results in a net energy storage cycle efficiency of only 50% at best. Such cells are further complicated by the need for a thermal management system because the difference in charging and discharging energy is dissipated as waste heat.
Attempts have been made toward the use of heat energy to directly drive regeneration of fuel cells. Osteryoung performed an extensive study toward this objective (see U.S. Pat. No. 5,208,112). However, attempts towards thermal regeneration have generally shown very limited success (see Chum, Helena L. and Osteryoung, Robert A., Review of Thermally Regenerative Electrochemical Systems, Solar Energy Research Institute; U.S. Department of Energy Contract No. EG-77-C-01-4042, Vol. 1 and 2, Task No. 3356.10 (August 1980)).
Accordingly, there remains a need for a practical, cost effective electrical power source that operates on heat and that can effectively respond to energy demand profiles in a manner that is independent of the limitations of its primary energy source profile.